1. Field of the Invention
The present invention relates to an automobile antenna system and more particularly, to an improved automobile antenna system for effectively detecting broadcast waves received by the vehicle body of an automobile and supplying a detection signal to various receivers installed in the vehicle body.
2. Description of the Prior Art
Antenna systems are indispensable to automobiles which must positively receive various broadcast waves such as those for radio, television and telephone at the receivers located within the vehicle. Such antenna systems are also very important for citizen band transceivers. Therefore, such antenna systems play an important role in the communication equipment which will be installed as a factory-installed item in the car of the future.
One of the conventional antenna systems is generally known as a pole antenna which projects outwardly from the vehicle body. Although such a pole antenna is superior in performance, it always remains a nuisance from the viewpoint of vehicle body design.
Furthermore, such a pole antenna is disadvantageous in that it is subject to damage, tampering or theft and also in that the antenna tends to generate noise during high-speed driving. For these reasons, there has heretofore been a strong desire to eliminate the need for such pole antennas.
With the enlargement of the frequency bands for broadcasting or communication waves received at automobiles in recent years, a plurality of pole antennas have been required in accordance with respective frequency bands. This brings about other problems; a plurality of pole antennas detracts from the aesthetic appearance of the automobile, and the receiving performance is greatly deteriorated by electrical interference between the antennas.
Various efforts have hitherto been made to eliminate the pole antenna system or to conceal it from the exterior. One such proposal has been to paste a small-gauge antenna wire on the rear windshield of an automobile, and this proposal has been put to practical use.
Another type of improved TV antenna system has been proposed which detects the surface currents induced on a vehicle body by broadcast waves. Although utilization of currents which flow on the vehicle may apparently be the most reliable and efficient means, experiments carried out heretofore have shown very unfavorable results.
The first reason why the surface currents on an ordinary vehicle body cannot be effectively utilized is that the value of the surface currents has proved to be lower than expected so that it has not been possible to obtain a detecting output from the surface currents on the roof panel of a vehicle body (the portion mainly used for detection) that is at a high enough level.
Another reason is that surface currents often have noise mixed therein. This noise is mainly produced by the engine ignition system of the vehicle and its battery charging regulator system, and it leaks into the vehicle body during the operation of the engine. It is therefore impossible to realize reception of broadcast waves in a clear enough manner for this system to be put to practical use.
In spite of such an unfavorable situation, some proposals have hiterto been made to overcome the above problems. One such proposal is disclosed in Japanese Patent Publication No. 22418/1978. In this antenna system, an electrical insulation is formed at a portion of the vehicle body on which currents are concentrated, with the currents being detected directly by a sensor between the opposite ends of the insulation. Although such structure can detect utilizable signals which are superior in SN ratio, a pickup used therein requires a particular cutout in the vehicle body. This cannot be accepted in the mass-production of automobiles.
Another proposal is disclosed in Japanese Utility Model Publication No. 34826/1978 in which an antenna including a pickup coil is provided for detecting currents flowing in a pillar of the vehicle body. This system is advantageous in that the antenna can be disposed completely within a vehicle body. However it is not practical for the pickup coil used therein to be located adjacent to the vehicle pillar in a direction perpendicular to the longitudinal axis of the pillar. Thus, it also appears that this arrangement cannot pick up any utilizable output of the antenna.
As has been described above, the conventional antenna systems have not been successful in efficiently detecting currents induced on the vehicle body by broadcast waves.
No effective measure has heretofore been proposed for overcoming the above-described problems of the conventional art in providing, in particular, a pickup structure for effectively detecting currents induced on the vehicle body by broadcast waves and a pickup arrangement capable of obtaining a utilizable SN ratio.
Furthermore, in the conventional antenna systems the receiving sensitivity is low and varies in accordance with a frequency band.